Directional filters (DFs) have gained interest in the applications of frequency division multiplexing and system stability improvement at circuit level. They may act either as channel combiners or channel separators. DFs address miniaturization and low reflection requirements of various implementations. Various DFs have been devised; however, a limited number of them have found practical applications.
FIGS. 1(a), 1(b), 1(c) and 1(d) show various examples of prior art DFs. For example, a broadband signal fed into Port 1 of the DFs shown in FIGS. 1(a)-(d) will be isolated at Port 4 (“isolated” indicating the signal is substantially not sent to Port 4), the desired spectrum will be dropped to Port 3, and the signal at the undesired spectrum will travel to Port 2, with substantially no reflection back to Port 1. According to mechanism, DFs can be classified into three categories: waveguide-based DFs, standing-wave DFs, and traveling-wave DFs. A waveguide-based DF comprises two rectangular waveguides and one cylindrical directly-coupled cavity resonator, as shown in FIG. 1(a). Typically, it is bulky and heavy, and has a narrow bandwidth of less than 2%.
FIGS. 1(b) and 1(c) show two types of standing-wave DFs with essentially the same frequency response. Each of them has two standing-wave resonators between two terminating lines, and can provide several percent bandwidth. However, tiny coupling gaps between the resonators and terminating lines are critically desired to provide sufficient coupling, which is extremely challenging for the commercially available circuit printing technologies, particularly when fabricating the devices for applications at high frequencies.
A traveling-wave DF comprises one or several traveling-wave loop resonators and two terminating lines, as shown in FIG. 1(d). The resonators and terminating lines are coupled by means of quarter-wavelength directional couplers. Passband width on the order of several percent can be achieved by using multiple loops. However, the traveling-wave DFs suffer from the same fabrication tolerance problem as the standing-wave DFs as the frequency increases.
The DFs of FIGS. 1(a)-(d) are typically designed to operate at a frequency below 40 GHz. As the frequency increases to W-band, i.e., 75-110 GHz, it becomes increasingly critical to obtain sufficient coupling between resonators and terminating lines with planar structures, leading to significant insertion loss. To this end, multilayer directional couplers were introduced to construct DFs. In these structures, the resonators and terminating lines are overlapped vertically, which may enhance the coupling, but may introduce large reflection and insertion loss. Insertion losses of the multilayer DFs may be as high as 5 dB at 6 GHz, 4 dB at 6 GHz, and 2.9 dB at 38 GHz. The coupling efficiency of the traditional multilayer quarter-wavelength directional coupler is strongly limited by the thickness of the substrates. Thus, it is challenging to scale these multilayer DFs to higher frequencies due to the limited available thickness of the circuit substrate.